1. Field of the Invention
The present invention relates to a frost sensor (frost detector), and more particularly to a frost sensor suitable for detecting frost deposited on a refrigerator (evaporator) of a refrigerating machine, a freezer, a room air conditioner or the like.
2. Description of the Prior Art
Optical and mechanical means have been known as means for detecting frost deposited on an article. The optical means detects the presence of frost deposited on the article by a difference of a light transmission or light reflection between a frosted state and a non-frosted state. Since the optical means can be constructed by a photo-transistor and a light emitting diode, the structure is simple. However, since the light transmission or light reflection changes when a surface of the article is contaminated or depending on the shape of frost deposited on the article surface, it is difficult to exactly detect the presence of frost. The mechanical means uses a contact probe (piece), for example, to detect the thickness of frost deposited on the article to detect the presence of frost. The mechanical means can directly detect the presence of frost but it must include means for moving the contact probe (piece) to adjust the position thereof. It is, therefore, complex in structure. It usually includes a movable member to detect the contact of the contact probe (piece) to the frost. Since the movable member is apt to be frozen by the frost, exact detection of frost is difficult. In addition, the adjustment of position of the contact probe (piece) is not easy.
Means for detecting frost deposited on a refrigerator of a room air conditioner is disclosed in the U.S. Pat. No. 3,466,888. In the disclosed detection means, the temperature of the refrigerator and the ambient temperature are sensed by a pair of thermistors to detect a change in the difference between those temperatures. It indirectly detects the presence of frost but does not detect the frost directly. Accordingly, it is difficult to precisely (with high sensitivity) detect the presence of frost.
Thus, the prior art frost sensors exhibit many disadvantages.